gitlab-ci.yml: install cargo-ndk for clippy step
This fixes #35 (closed). (I didn't do this originally because I didn't expect that the ./build-ndk.sh script would install cargo-ndk, and so assumed it'd be in the Docker image!)
This fixes #35 (closed). (I didn't do this originally because I didn't expect that the ./build-ndk.sh script would install cargo-ndk, and so assumed it'd be in the Docker image!)